Memory controller

ABSTRACT

The present invention relates to a memory controller for an IC with an external DRAM, where the external DRAM has at least one memory bank and communicates with the IC via at least one channel. In line with the invention, the memory controller has a command scheduler which prioritizes the transmission of memory bank commands on the basis of a static priority allocation for commands and a dynamic priority allocation for channels.

The present invention relates to a memory controller for an IC with an external DRAM, particularly a memory controller with a command scheduler.

Statistics for the greatest sales of pre-recorded image data storage media in Germany show that the DVD replaced the video cassette for the first time last year. It can thus be assumed that in the foreseeable future DVD video recorders will replace analogue video recorders as a recording unit. Particularly digital television reception (DVB), which is currently spreading quickly, will also awaken the desire for digital storage. In this context, however, it has already been found that even modern compression methods such as MPEG-4 allow only two cinema films, on average, to be stored on a writeable DVD. As high-definition television (HDTV) progresses, the development of new optical storage media with a greater storage capacity is therefore being pushed ahead. An example of an optical storage medium of this type is the Blu-ray disc with a capacity of up to 54 GB. Future drives for optical storage media should preferably support at least the formats of Blu-ray disc, DVD and CD both for reading and for writing. To this end, an IC for controlling the optical drive is required which can handle said formats. In the field of image processing, too, e.g. in DVB receivers, powerful ICs are required.

To reduce the costs of digital end-consumer appliances, monolithic IC solutions (“one-chip solutions”) are being used today, wherever this is possible. This means that embedded central processing units (CPUs) and/or digital signal processors (DSPs) are used instead of dedicated hardware in order to reduce the development time for the systems. To store instructions and data from these processors and to buffer the data stream from a drive or a video data stream, large memories of up to several megabytes are needed. To keep down the costs of the memory, DRAM (dynamic RAM) is normally used instead of SRAM (static RAM). Preferably, an SDRAM (synchronous DRAM) is used for the DRAM, or else other types of DRAM are used, such as DDR-RAM (double data rate RAM), ESDRAM (enhanced synchronous DRAM), SLDRAM (synchronous link DRAM) or RDRAM (Rambus DRAM). The text below refers either to DRAM generally or to SDRAM specifically. It goes without saying that the invention is not limited to the use of SDRAM.

Embedded DRAM is relatively expensive and IC processes which support embedded DRAM are rare. For this reason, external DRAM is normally used. To keep down the costs for the IC development, a limited number of pins are used, particularly in order to achieve a small physical size for the IC. On account of this limitation, the external data bus to the DRAM is often narrower than the internal data bus. This results in a bottleneck. In addition, the DRAM is used for storing a wide variety of data, i.e. it is used jointly by the available CPU DSP and real-time data streams. This aggravates the bottleneck additionally.

Typical SDRAM modules comprise four independent memory banks. Each memory bank comprises rows, which in turn comprise columns. In order to address a specific data value, the appropriate row in the appropriate memory bank first needs to be activated (‘activate’). Following activation, which takes between two and four clock cycles, the data transfer can be initiated by transmitting a read or write command together with the desired column address. Following the data transfer, the memory bank is ‘precharged’ in order to deactivate the open row and to prepare the memory bank for the next activation command. ‘Precharge’ especially means that a memory address is already being prepared for access, because the system knows from the outset that a request to this address will be sent in the next clock cycles.

Normally, a burst transfer is used in order to read or write a plurality of data values using just a single read or write command. The access starts at a prescribed location (column) and is continued over a programmed number of locations. When a new burst has been initiated, the command bus is free and may be used for activating or precharging other memory banks.

The memory banks are thus controlled independently of one another, but share the same command lines. Only one command can therefore be sent in each clock cycle.

To manage with the smallest possible number of pins and a small physical size, the data throughput on the external data bus to the DRAM module needs to be maximized. A problem in this context is that the DRAM, as explained above using the example of SDRAM, requires a plurality of clock cycles for activating rows and for ‘precharging’ memory banks. This results in waiting times of several clock cycles between the data transfers. Depending on the length of the read or write bursts, these waiting times may result in more clock cycles remaining unused overall than clock cycles being used for the data transfer. An example of this is provided by FIG. 1 a), which shows two write bursts over four respective clock cycles. Between the write bursts, there are seven clock cycles without any data transfer. To conceal the waiting times, the next data transfer already needs to be in preparation while a read or write burst is in progress. An example of this is shown in FIG. 1 b), which likewise shows two write bursts over four respective clock cycles. The waiting times are concealed behind other data transfers.

It is known practice to eliminate the bottleneck by using a wider external data bus to the DRAM module or alternatively an embedded SRAM in order to reduce the workload on the external databus in this manner. Both solutions are relatively expensive to implement.

It is an object of the invention to propose a memory controller which allows a high data throughput with reduced waiting times and is inexpensive to implement.

The invention is explained below using the example of a single DRAM module. It is likewise possible to use a plurality of memory modules by connecting all memory modules to the same data bus and to the same command bus. In this case, a chip enable signal is used in order to select the desired module.

To use one or more DRAM modules jointly for a plurality of applications, it is necessary to ensure that the memory areas for the various processors and for the real-time data stream are physically separate. This means that all have one or more especially associated DRAM memory banks. This requirement may be dispensed with if it is ensured that no successive access operations to a jointly used memory bank arise.

In line with the invention, a method for communication between an IC and an external DRAM, where the external DRAM has at least two memory banks and communicates with the IC via at least one channel, involves the transmission of memory bank commands being prioritized on the basis of a static priority allocation for commands and a dynamic priority allocation for channels.

It is advantageous to the invention if the states of DRAM memory banks are depicted by associated state machines. This makes it possible to control all memory banks independently of one another. For each access operation, the state machines receive the type of transfer (read or write), the row number and the column number. By observing particular rules for time coordination, they control the memory banks by sending commands to the command scheduler. In this case, each channel is connected to the state machine which controls the associated memory bank. If a channel is able to access a plurality of memory banks, a network is required. The command scheduler ensures that the same memory bank is not addressed a plurality of times in succession. Between two access operations to a memory bank, an access operation to another memory bank is always effected. Alternatively, however, two successive access operations to a memory bank are permitted if they are made to the same row in the memory bank, which means that no waiting times arise as a result of the activation or precharging. The priority allocation sorts the pending commands according to their ability to start a new burst in such a way that optimum use of the DRAM data bus is achieved. This means that read and write commands have a high priority, followed by activation commands, which are a prerequisite for read or write commands. Precharging commands are given the lowest priority, since they are not part of the current transfer. Precharing commands are required only for successive transfers, and they can therefore be delayed. If all bursts have a length of four or more clock cycles, the workload on the command lines is small enough to transmit commands with low priority without a long delay. To stipulate the order of the waiting commands, the commands need to be analysed, grouped and sorted according to their ability to start a data transfer as quickly as possible: Command Priority Burst Terminate 4 (highest) Read or Write Burst 3 Activate 2 Precharge 1 (lowest)

The invention causes the DRAM module's bandwidth attained to be close to the physical maximum. With a large workload, the latencies are hidden entirely by Activate and Precharge, which means that the access times continue to be short. The demanding object of allowing access operations with low latency for some channels and of simultaneously ensuring a high data throughput for other channels is achieved very well by the inventive memory controller.

In line with a further aspect of the invention, a memory controller for an IC with an external DRAM, where the external DRAM has at least two memory banks and communicates with the IC via at least one channel, has a command scheduler which prioritizes the transmission of memory bank commands on the basis of a static priority allocation for commands and a dynamic priority allocation for channels.

Advantageously, an appliance for reading and/or writing to optical storage media has an inventive memory controller or uses an inventive method for communication between an IC and an external DRAM.

To improve understanding, the invention will be explained below with reference to FIGS. 1 to 5. In this case, identical reference symbols denote identical elements. It goes without saying that the invention is not limited to the exemplary embodiments shown. Features of the invention may readily be combined or modified without departing form the scope of validity of the invention. In the figures:

FIG. 1 shows two examples of two write bursts over four respective clock cycles;

FIG. 2 shows the block diagram of an inventive memory controller;

FIG. 3 shows the block diagram of a command scheduler;

FIG. 4 shows a state diagram of the priority allocation; and

FIG. 5 shows a flowchart for the command scheduler.

FIG. 2 shows the block diagram of an inventive memory controller using the example of an SDRAM controller 1 in a system with three channels: a CPU connected via an AMBA bus 8, and also an input 6 and an output 7 for a real-time data stream. Each memory bank 21, 22, 23, 24 in the SDRAM module 2 has an associated state machine 41, 42, 43, 44 in a memory bank control unit 4, which depicts the respective state of the memory bank 21, 22, 23, 24 and is responsible for observing the waiting times and the correct state sequence. These state machines 41, 42, 43, 44 transmit their commands for the memory banks 21, 22, 23, 24 to a command scheduler 3 (command bus scheduler) which watches over the allocation of the external command and data bus. In each clock cycle, the command scheduler 3 transmits a command selected according to priority to the DRAM module 2. The state machines 41, 42, 43, 44 obtain their transfer orders directly from the three channels (input 6, AMBA 8 and output 7), which are forwarded by a memory bank scheduling unit 5 on the basis of their address and priority to the appropriate memory bank 21, 22, 23, 24. The memory bank scheduling unit 5 contains a network in order to allow all channels 6, 7, 8 to access all memory banks 21, 22, 23, 24. During a read access operation to the storage medium, the input channel 6 accepts the data from an ECC (Error Correction Code) unit (not shown), and the output channel 7 forwards the data to an ATAPI block (not shown). Both channels 6, 7 contain FIFOs (not shown) in order to prevent the flow of data from being held up. During a write access operation to the storage medium, the input channel 6 receives the data from the ATAPI block, and the output channel 7 forwards them to the ECC unit. The AMBA (Advanced Microcontroller Bus Architecture) channel 8 comprises an AMBA slave, which additionally permits access to a register file 34 as well (see FIG. 3). It contains a read cache and a write cache (not shown) in order to reduce the blocking time for the AMBA bus. Since the internal state of each of the four SDRAM memory banks 21, 22, 23, 24 is depicted by a separate state machine, merely accessing a state machine 41, 42, 43, 44 may result in competition by the three channels 6, 7, 8 which is not handled by an upstream scheduling algorithm. However, it is possible to ensure that this competition situation arises only rarely. For this reason, the real-time data stream of the sector data is granted priority over the ARAM access operations in this case.

A more detailed block diagram of the command scheduler 3 is shown in FIG. 3. The incoming commands from the state machines 41, 42, 43, 44 which manage the four memory banks 21, 22, 23, 24 are analysed by a command analyser 31. In this case, it is ascertained for each of the five possible commands—Activate, Read, Write, Precharge and Burst Terminate—whether it is present at least once. If this is the case, the first occurrence is sent to a scheduler 32. It is thus only relevant whether a command is waiting; the memory bank 21, 22, 23, 24 or the currently associated channel 6, 7, 8 is of no significance. The scheduler 32 has access to a register file 34 which contains particular operating parameters for the scheduler 32.

The scheduler 32 itself first checks the presence of a global command. If a command for programming the mode register, which contains the operating parameters DRAM, is present for a global refresh or a global precharge, then it is executed directly. Since the memory bank control unit 4 ensures that a global command can arise only when the memory banks 21, 22, 23, 24 are in the idle state, no further check is required here.

If the global command sent is an NOP (No Operation), then a memory bank command can be transmitted to the DRAM module 2. Memory bank commands are transmitted on the basis of a static priority allocation for commands and a dynamic priority allocation for channels 6, 7, 8, which are executed by a priority allocation unit 33. In this case, the priority of commands is higher than that of the channels 6, 7, 8. This means that first a command type is chosen and, if there are a plurality of channels 6, 7, 8 which wish to send this command, then the channel 6, 7, 8 which is allowed to send the command is chosen.

Among the commands, the Burst Terminate command for terminating bursts has the highest priority. The Read and Write commands have the second highest priority, since they initiate a new burst and sending them as quickly as possible is thus the prerequisite for a good utilisation level for the data bus. The next lowest priority is held by the Activate command, which is used for opening a row. Since the opening of a row is a prerequisite for starting a burst, the priority of the Activate command is above that of the Precharge command, which has the lowest priority, since it is executed after a burst has been terminated and, provided that no subsequent access is pending, does not influence the overall performance.

The dynamic prioritisation of the channels 6, 7, 8 is effected by an algorithm as is shown in the form of a final state machine, i.e. a state diagram, in FIG. 4. The algorithm shown controls the access operations of a CPU via an AMBA-AHB (Advanced High-performance Bus) and a real-time data stream with its two channels (input and output). The states represent the priority levels. In this case, the channel shown in the top state has the highest priority and the bottom channel has the lowest priority. The state transitions represent the channel which can ultimately start a read or write burst. A possible additional channel for a flash controller, for example for sending firmware etc., is not included, since it does not compete with the other channels.

As can be seen, the Input→AMBA→Output→AMBA sequence is always observed when all three channels are active. If the channel with the highest priority is not able to send a command, since it is currently not active or its command has too low a priority, the channel which can ultimately send its command is given the lowest priority in the next clock cycle. At the same time, however, it is ensured that the AMBA channel 8 is given the highest priority in the next clock cycle if it does not have the highest priority in the current clock cycle and another channel has an opportunity. Once the AMBA channel 8 has been given the highest priority, it loses it again only when it can send a command. This ensures the lowest possible latency for the ARM. The state diagram shown guarantees short delay times for the CPU access operations, since the AMBA channel 8 is given the highest priority after every burst via the input channel 6 or the output channel 7. In addition, the state diagram ensures fair use of the data bus and alternating access operations to the memory banks 21, 22, 23, 24. The algorithm is designed for CPU access operations with high priority and hence low latency given simultaneous guaranteed data throughput for the real-time data stream. The data throughput is stipulated by the length of the read and write bursts through the input channel 6 and the output channel 7 upon data transfer from and to the DRAM 2.

The decision regarding which channel is permitted to send its command will be explained below with reference to the flow chart shown in FIG. 5. If a memory bank FSM (Final State Machine) wishes to terminate its burst, this has the highest priority. If, consequently, a Burst Terminate (BST) has been found after the start 9 during the analysis 10, the burst in progress is aborted. This can be done in two ways: first by simply forwarding 12 the Burst Terminate command, secondly by starting 14 a new burst. Before a Burst Terminate is now sent, a check 11 is performed to determine whether a Read or Write command is likewise waiting to be sent. If this is the case, this command is sent 14 instead of the Burst Terminate. During the analysis, there is merely a check to determine whether at least one Read or Write command is present. For this reason, before the Read or Write command is sent 14, the dynamic priority allocation for the channels is used to check which is the channel which has a Read or Write command and the highest priority. This channel is then permitted to send its command. The choice of channel is communicated to the priority allocation, which thus changes to a new state with a new distribution of priorities in the next clock cycle. The only restriction for replacement of the Burst Terminate command with a Read of Write command is that a Read Burst cannot be terminated by a Write Burst, since otherwise the memory controller 1 and the DRAM module 2 are driving the data bus simultaneously. When a lower clock frequency is used, this restriction can be bypassed, however, since the hold time for the outputs of the DRAM is constant and is not dependent on the clock frequency.

If no Burst Terminate command is present, then the presence of Read or Write commands is checked 13 and, if they are present, the command is transmitted 14 on the basis of the priority allocation. If no Read or Write commands are present either, then there is a check 15 for Activate commands. If an Activate command of this type is waiting, it is transmitted 16. If there is no Activate command, there is a check 17 for Precharge commands. Any Precharge command which is present is transmitted 18. Should no channel 6, 7, 8 or no memory bank 21, 22, 23, 24 wish to send a command, an NOP (No Operation) is transmitted. If a command is sent to the DRAM module 2, the memory bank FSM from which this command comes is informed by a signal, as a result of which it changes to a new state in the next clock cycle.

In order to observe the Set-up and Hold times of the DRAM module 2 when sending commands, the DRAM module 2 is preferably operated using an inverted system clock. The commands and the data for a Write access operation are thus accepted by the DRAM module 2 with a delay of half of one clock cycle plus the signal propagation time for the input and output drivers and the signals propagation time on the circuit board. 

1. Method for communication between an IC and an external DRAM, where the external DRAM has at least one memory bank and communicates with the IC via two or more channels, wherein the transmission of memory bank commands is prioritized on the basis of a static priority allocation for commands and a dynamic priority allocation for channels.
 2. Method according to claim 1, wherein the static priority allocation for commands involves a ‘Burst Terminate’ command being given the highest, a ‘Read’ or ‘Write’ command being given the second highest, an ‘Activate’ command being given the third highest and a ‘Precharge’ command being given the lowest priority.
 3. Method according to claim 1, wherein the dynamic priority allocation for channels involves a channel being given the lowest priority after an command has been sent.
 4. Method according to claim 1, wherein the dynamic priority allocation involves one of the channels being given the highest priority in the next clock cycle if it does not have the highest priority in the current clock cycle and another channel sends a command.
 5. Method according to claim 1, wherein the dynamic priority allocation may involve one of the channels losing the highest priority only when it can send a command.
 6. Method according to claim 1, wherein the channels access physically separate memory areas in the external DRAM.
 7. Method according to claim 1, wherein the channels access jointly used memory areas in the external DRAM and the assurance is given that no successive access operations to a jointly used memory area will arise.
 8. Method according to claim 1, wherein a network is provided which allows at least one channel to access various memory banks.
 9. Method according to claim 1, wherein two access operations to a memory bank always have an access operation to another memory bank effected between them.
 10. Method according to claim 1, wherein two successive access operations to a memory bank are permitted when they are made to the same row in the memory bank.
 11. Method according to claim 1, wherein the states of the memory banks are depicted by associated state machines.
 12. Method according to claim 1, wherein a plurality of DRAM modules are used and a chip enable signal is transmitted in order to select the desired module.
 13. Memory controller for an IC with an external DRAM, where the external DRAM has at least one memory bank and communicates with the IC via two or more channels, wherein it has a command scheduler which prioritizes the transmission of memory bank commands on the basis of a static priority allocation for commands and a dynamic priority allocation for channels.
 14. Appliance for reading and/or writing to storage media, wherein it uses a method according to claim 1 or has a memory controller. 